JP-2008-008380A describes a spool valve having a damper which restricts a spool from having rapid movement so as to suppress rapid variation in an oil pressure. Specifically, a damper chamber defined between the spool and a sleeve is made to communicate with outside of the sleeve through a small-diameter orifice. The damper chamber may also be referred as a spring chamber, and a space volume of the damper chamber is varied when the spool moves in an axis direction. In a state where the damper chamber is filled with oil, oil passes through only the orifice and the volume of the damper chamber is varied, such that the movement speed of the spool is controlled.
In JP-2008-008380A, a part of a valve body in which the spool valve is disposed is located and used in oil. If the spool valve is used in air, air may be mixed into oil in the damper chamber when oil flows out of the damper chamber, and air may pass through the orifice when the damper chamber has a volume variation. In this case, the damper effect may not be obtained due to the air.